Graphene and graphene based nanomaterials are one of the most intriguing functional materials today because of their exceptional properties including unique electronic properties and high mass to strength ratio, etc. Their unique properties, plus the abundance of naturally existing graphite, the ease of functionalization and synthesis in mass volume; graphene and graphitic nanomaterials have shown great potential in multifunctional polymer nanocomposites. Graphite Nanoplatelets (GNPs) is one of the most desirable graphitic nanomaterials as the reinforcing component for polymer nanocomposites, due to its low cost and higher specific surface area of GNPs compared to other graphitic nanomaterials. In this study, the heat treated GNPs (ht-GNPs) and surface modified ht-GNP (s-GNP) were used to modify ultrahigh molecular weight polyethylene (UHMWPE). The objective of this study focused on understanding the effects of the GNP loadings, temperature, as well as electric field on energy storage capability of UHMWPE/GNP nanocomposites with and without surface modifications of ht-GNPs. The results suggested that the surface modification could remarkably improve the dispersion of GNPs and the interfacial interactions between GNPs and UHMWPE, both factors have shown the positive impacts on the energy storage performances regardless of testing conditions, compared with ht-GNPs without surface modifications. Meanwhile, at the elevated temperature, for both UHMWPE and nanocomposites, both energy storage capability and efficiency increased, probably due to enhanced dipole switching at high temperature and high electrical field. The results in this research also revealed the sensitivity of the energy storage of UHMWPE/GNPs nanocomposites to various factors, which provides us great opportunities to further optimization of energy storage performances, and eventually leads to high performance energy storage materials based on polymer nanocomposites.